Ectasia/Forward shift (bulging) of the cornea after LASIK

Matched Population Comparison of the Visian Implantable Collamer Lens and Standard LASIK for Myopia of ?3.00 to ?7.88 Diopters

Journal of Refractive Surgery Vol. 23 No. 6 June 2007

Donald R. Sanders, MD, PhD

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Review of the LASIK literature for ectasia incidence was also performed. Many of the studies had small to medium patient populations (from 15 to 300 LASIK cases) with no report of ectasia,34-43 although the study with the largest patient population of 2873 LASIK cases reported an ectasia incidence of 0.66% (19 cases).44 The present LASIK group reported 1 (0.6%) case with ectasia. Ectasia has no single etiology, and cases with no apparent risk factors have been found to develop ectasia. To minimize ectasia incidence, careful screening of patients should exclude those with preexisting keratoconus/forme fruste keratoconus, abnormal topography, and thin corneas.

_________________Broken Eyes

"The price good men pay for indifference to public affairs is to be ruled by evil men." Plato

The importance of corneal biomechanics in refractive surgery outcomes has become apparent with recent publications.11,12 The corneal stroma consists of lamellae (organized collagen fibers), which run from limbus to limbus. Traditional LASIK, using a mechanical microkeratome, creates a fl ap approximately 160 μm thick, which severs a significant number of collagen fibers compared to PRK. The loss of lamellar integrity following LASIK results in compromised corneal biomechanical integrity due to minimal biomechanical loading distributed throughout the flap. Hence, there is no contribution from the flap to the biomechanical stability of the cornea.1 Cohesive tensile strength studies demonstrate that Bowman?s layer is the strongest structural component of the cornea followed by the anterior third of the corneal stroma.1,13 In fact, the peripheral anterior third of the corneal stroma is stronger than the paracentral and central anterior third.12 These findings are supported by morphologic studies that demonstrate more collagen lamellar interweaving and collagen lamellae orientations that were transverse to the anterior surface of the cornea.14-16 These studies suggest that a thin, uniform flap would leave more of the strong anterior stroma untouched, which should provide greater corneal biomechanical strength than the thicker traditional LASIK flap that severs more of these strong anterior fibers.

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In addition, we speculate that flaps made deep to this level (traditional LASIK), in the weaker posterior cornea where the lamellae lie more parallel and less compact, create weaker corneal biomechanics.

_________________Broken Eyes

"The price good men pay for indifference to public affairs is to be ruled by evil men." Plato

Given that the corneal stroma consists of lamellae thought to run from limbus to limbus across the corneal arc and that the lamellae consist of organized collagen fibers, loss of lamellar integrity may compromise corneal strength. A significant number of collagen fibers are severed in LASIK compared with a corresponding PRK procedure.4 Hence, the microkeratome flap in LASIK uncouples a significant proportion of the corneal biomechanics, which may affect refractive stability. With the significant reduction in the biomechanical integrity of the cornea, at worst LASIK has the risk for inducing iatrogenic keratectasia5, 6, 7, 8, 9, 10, 11 and at best, a propensity for long-term instability.

Although the incidence of iatrogenic keratectasia appears low at present, LASIK has only been used in clinical practice in recent years and the long-term results are unknown. Despite this potentially serious complication, the popularity of LASIK is growing at a fast pace and more than 5.5 million excimer laser refractive procedures have been performed. Although iatrogenic keratectasia is most common in eyes in which thin beds have been left, cases have been observed when the corneal bed after surgery is thicker than 250 μm, which is regarded by some as an arbitrary safe minimum corneal thickness.

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When the microkeratome flap was replaced to cover the stromal bed, the out-of-plane surface movement was of the same order as the movement when the flap was removed, exposing the stromal bed. This indicates that although tissue bulk had been restored, the biomechanical properties remained altered relative to preoperatively because of uncoupling of the collagen fibril array.

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The study showed that measurable changes occur in corneal displacement subsequent to microkeratome incisions and such changes are hardly influenced by relocation of the flap. Furthermore, such variations were measured with pressure changes of 0.15 mm Hg (20 Pa), corresponding to a 1% change in IOP. The apparent small difference in out-of-plane displacement (0.3 μm) between the operated and unoperated eyes should be considered in relation to the precision of the measuring technique; that is, 0.01 μm. Thus, these are real effects and should be of concern to the ophthalmic community.

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Our results can be summarized as showing (1) a measurable change in out-of-plane forward movement or ?bulging? of the cornea with increasing pressure and (2) a region of disharmony coincident with the plane of the microkeratome transsection.

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In contrast, biomechanical considerations would predict instability with LASIK. This concept is derived from the finding that no repair transgresses the plane of the microkeratome incision and integrity in this region is established by deposition of ground substances such as fibronectin and tenascin. It is further supported by histological observation of corneas that have keratoplasty after LASIK. In such samples, although the bed remains intact, the collagen fibers within the flap show disorganization and atrophy. This finding supports the concept of reduced strain in the flap because of uncoupling from the stromal bed and atrophy of the fibers tectonically isolated from biomechanical movements engendered by processes such as accommodation. It is unfortunate that few studies have been published on the long-term effects of LASIK. The 1 study that reports 6-year data gives graphic information that is contrary to written claims in that there is a trend toward regression with time. If this trend becomes significant, it further emphasizes the need for a better understanding of the biomechanics of the cornea.32

_________________Broken Eyes

"The price good men pay for indifference to public affairs is to be ruled by evil men." Plato

The posterior corneal elevation increased significantly after LASIK (flap creation + ablation) compared to that after partial flap (without ablation). Hence, residual bed thickness after ablation may be the sole determinant for the increase in posterior corneal elevation following LASIK. Photoablation results in expulsion of tissue fragments from the target site, which occurs within microseconds or less.16 This process generates reactive forces within the cornea and stress waves of the amplitude up to 100 atm at the level of cornea,17,18 compromising the structural integrity of cornea. This may possibly increase the posterior corneal elevation following LASIK, which is attributed to the instantaneous biomechanical change in which the IOP pushes against the back surface of a structurally compromised cornea.

RESULTS: A patient 3 years post-LASIK developed bilateral corneal ectasia worse in the right eye following airbag deployment in a road traffic accident. At last follow up, best corrected vision was 20/40 with -4.00/-4.00x25 in the right eye and 20/25 with -1.25/-0.50x135 in the left eye.

CONCLUSIONS: This is a rare presentation of trauma induced ectasia in a patient post-LASIK. It is possible that reduction in biomechanical integrity of the cornea from prior refractive surgery contributed to this presentation.

_________________Bill

"What concerns me is that if the person informing the patient is themselves poorly or inaccurately informed then how on earth can consent ever be truly informed?" Dr. Sarah Smith

Collagen crosslinking procedures may prove to be effective for postoperative ectasia; however, most of the crosslinking effect occurs in the anterior stroma, a region of the cornea that is functionally decoupled from the posterior stroma after creation of the LASIK flap. Thus, the full potential effect of collagen crosslinking for postoperative ectasia remains to be determined. Collagen crosslinking is currently not approved by the U.S. Food and Drug Administration.

_________________Broken Eyes

"The price good men pay for indifference to public affairs is to be ruled by evil men." Plato

PURPOSE: To evaluate the corneal topographic characteristics of first-degree relatives of patients with keratoconus with corneal topography to determine the incidence of clinical keratoconus and topographic abnormalities.

METHODS: Between February and August 2006, Orbscan II analysis was done in 144 eyes of 72 cases who were first-degree relatives of patients diagnosed with clinical keratoconus. The findings were compared with preoperative Orbscan analyses of 52 clinically normal individuals who underwent laser in situ keratomileusis surgery and did not develop corneal ectasia in 3 years of follow-up.

RESULTS: In 8 of the 72 first-degree relatives of patients with keratoconus, clinical keratoconus was diagnosed by the topographic pattern in Orbscan and clinical examination (group 1). The remaining 64 subjects (group 2) were compared with the control group (group 3). The central corneal thickness was 523.7 +/- 40.4 microm in group 2, whereas it was 546.3 +/- 33.1 microm in group 3 (P < 0.05). The central corneal thickness, thinnest pachymetric reading, posterior elevation value, distance between the greatest anterior/posterior elevation points, and corneal center, posterior best fit sphere (BFS) values, posterior BFS:anterior BFS ratio, and irregularity values were significantly different between group 2 and group 3 (P < 0.05).

CONCLUSIONS: The keratoconus incidence was found to be 11% in first-degree relatives of patients with keratoconus as opposed to a reported incidence of keratoconus of 0.05% in the general population. In first-degree relatives of patients with keratoconus who did not have a topographic keratoconus pattern, abnormal corneal topographic values were detected. The asymptomatic relatives of patients with keratoconus should undergo a thorough preoperative analysis for subtle topographic abnormalities before any keratorefractive surgery.

_________________Broken Eyes

"The price good men pay for indifference to public affairs is to be ruled by evil men." Plato

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